1. Field of the Invention;
This invention relates to the control of noise which is propagated along a duct, for example by the action of a fan associated with the duct.
2. Description of Related Art
It has long been known that the level of such noise can be reduced by locating a receiving transducer, such as a microphone, to sense the noise at a position in the duct, locating a transmitting transducer, such as a loudspeaker, to introduce sound into the duct, amplifying and changing the phase of the output of the microphone and feeding the resulting signal to the loudspeaker so that the sound introduced into the duct by the loudspeaker destructively interferes with the noise at the microphone position. The noise level at that position, and downstream of that position, is thereby reduced.
A problem with such basic active noise control systems is that a filter or other compensating means must be used in the circuit which feeds the loudspeaker, in order to compensate for the characteristics of the feedback path within the duct, between the loudspeaker and the microphone, and this compensating means needs adjustment to compensate for changes in the feedback path to obtain the best noise attenuation which can be achieved while ensuring that the system does not howl.
It has previously been proposed to provide a feedback "management" system in which a second microphone is located downstream of the loudspeaker to pick up any residual noise, and thereby to produce a signal which is then used to control the feedback circuit to minimise the residual noise. This will detect any tendency for the feedback circuit to howl, and will operate to counteract it.
In the previous active noise control systems in which such management has been effected, the whole of the feedback loop has been digitally controlled. This has required the microphone and the loudspeaker to be displaced relative to each other along the duct by such a distance as will correspond to the time delay which is caused by the digital processing circuitry. The fact that the whole of the feedback system and the management system has operated digitally has meant that expensive digital circuitry is required.